Rabble tooth



March 30, 1965 G. R. GRIMES 3,175,809

RABBLE TOOTH Filed March 6, 1962 2 Sheets-Sheet l I I INVENTOR. /,M I eorfe 77'. 67777766.

March 30, 1965 e. R. GRIMES 3,175,809

RABBLE TOOTH Filed March 6, 1962 2 Sheets-Sheet 2 E Z I United States Patent 3,175,809 RABBLE TGOTH George R. Grimes, Burgettstown, Pa., assignor to American Metal Climax, line, New Yorlr, N.Y., a corporation of New York Filed Mar. 6, 1962, Ser. No. 177,842 8 Claims. (Cl. 259-126) The present invention broadly relates to multiple hearth rotary furnaces of the type employed for heating or roasting various materials such as ores, for example, and more particularly to an improved composite rabble tooth construction employed in such multiple hearth rotary furnaces which provides for substantially increased wear resistance and durability achieving thereby a substantial reduction in maintenance and downtime as heretofore required in effecting frequent replacement of worn rabble teeth.

Multiple hearth furnaces of the general type to which the present invention is applicable have been in use for many years for the high temperature processing of various materials such as for the roasting of ores, for the drying and calcining of lime, magnesite, and carbonate sludges; for the reactivation of decolorizing earths; for the burning and roasting of sulfides to produce sulfur dioxide, and the like. Various modifications f the basic rotary multiple hearth furnaces are in use and are known under various names such as the Herreshofi, McDougall, Wedge, Nichols, etc. The furnaces basically comprise a plurality of annular-shaped hearths mounted in vertically spaced relationship on which the material being processed is transferred from the uppermost hearth in cascading fashion downwardly to the lowermost hearth while being exposed to a counter-current flow of hot flue gases effecting a heating or roasting of the material. Movement of the material is achieved by rotatable scraper assemblies or rabble arm assemblies disposed above each hearth incorporating a plurality of scrapers or rabble teeth thereon that in response to rotation of the rabble arm assembly effect a successive radially inwardly and outwardly movement of the material from one hearth to the next hearth below in a cascading manner.

From the foregoing, it will be apparent that the conditions to which the rabble teeth are subjected within the furnace namely, the high temperatures, the corrosive atmosphere, and the abrasive contact with the hot materials being processed, is conducive to a rapid wear and deterioration of the rabble teeth necessitating a frequent replacement thereof in order to achieve a satisfactory flow of materials. The progressive deterioration and wear of the rabble teeth occur predominantly along the surfaces and edges in contact with the hot abrasive material being processed and is evidenced by a decrease in the size of the rabble tooth accompanied by a corresponding buildup of the depth of processing material on the hearth. When the wear has attained a magnitude whereby the material on the hearth has attained an objectionable depth, the rabble tooth must be replaced to avoid fusion or solidification of the thick heated layer of material into a nonfriable mass on the hearth.

To facilitate replacement of the rabble arms and the rabble teeth, various mounting means have been provided for removably mounting the rabble arms and the rabble teeth themselves in order that they can be removed when an objectionable degree of wear has occurred and replaced by new unworn components. Because of the long time required to place a rotary multiple hearth furnace of this type on stream which frequently requires several days to bring it up to the proper heat and operating conditions and because of the long time required to cool a furnace of this type, changes of the individual rabble teeth and rabble arms are conventionally made during the operation ice of the furnace to avoid long periods of unit downtime and to maintain a reasonable operating efficiency. The replacement of the individual rabble teeth under high temperature operating conditions is an extremely difficult, time-consuming and costly maintenance operation and has heretofore represented a continuing unresolved problem in furnaces of this type.

It is accordingly a primary object of the present invention to provide an improved composite rabble tooth construction which has a substantially greater resistance to wear and deterioration under the hot corrosive and abrasive conditions to which it is subjected necessitating substantially less frequent replacement while concurrently providing for a more uniform and consistent transfer of material through the furnace thereby substantially overcoming the problems heretofore encountered in multiplehearth furnaces of similar type.

Another object of the present invention is to provide an improved rabble tooth which because of its greater resistance to Wear and deterioration under the furnace operating conditions, substantially reduces the frequency of processing interruption as heretofore required for replacement of worn rabble teeth resulting therein in a substantial improvement in the overall operating efiiciency of the furnace.

Still another object of the present invention is to provide an improved rabble tooth construction incorporating therein a hard, wear-resistant insert which is adapted to be positioned adjacent the edges exposed in hot, abrasive contact with the heated material being moved thereby providing increases in the useful operating life of the rabble tooth in the order of about eight times that of rabble teeth heretofore known under similar operating conditions.

A further object of the present invention is to provide a rabble tooth with a hard, wear-resistant insert disposed within a cast iron body and mechanically interlocked therein.

A still further object of the present invention is to provide an improved rabble tooth construction which is of simple design, of efiicient and durable operation, and of simple and economical manufacture.

The foregoing and other objects and advantages of the present invention are achieved by forming a rabble tooth comprising a cast iron body member including a blade portion having a hard, wear-resistant insert therein which is of a relatively fragile nature but is prevented from shattering or breaking by virtue of the protective support of the overlying cast iron body. The composite rabble tooth comprising the present invention can be simply and economically manufactured by centrally positioning a hard, wear-resistant insert of the appropriate size and configuration having a removable positioning segment within a mold cavity including a positioning slot and thereafter pouring molten iron therearound which on solidification forms a tough supporting structure about the relatively fragile insert and is interlocked therewith preventing separation or loss of the insert.

Other objects and advantages of the present invention will become apparent from the following detailed descripion taken in conjunction with the accompanying drawings, wherein:

FIGURE 1 is a fragmentary side elevation view partly in section of a portion of a typical multiple-hearth rotary furnace to which the improved rabble tooth construction comprising the present invention is applicable.

FIG. 2 is a transverse horizontal sectional view taken through the multiple-hearth furnace shown in FIGURE 1 and taken along the lines 22 thereof;

PEG. 3 is a plan view of a rabble tooth constructed in accordance with one embodiment of the present invention;

FIG. 4 is a diagonal vertical sectional view of the rabble tooth shown in FIGURE 3 and taken along the line 4-4 thereof;

FIG. 5 is a transverse vertical sectional view through the rabble tooth shown in FIGURE 3 and taken along the line 55 thereof;

FIG. 6 is a bottom plan view of the scraping edge of the rabble tooth shown in FIGURE 4 as viewed along the line 66 thereof;

FIG. 7 is a plan view of a terminal-type rabble tooth constructed in accordance with a second embodiment of the present invention;

FIG. 8 is a diagonal vertical sectional view through the rabble tooth shown in FIGURE 7 and taken along the line 8-3 thereof;

FIG. 9 is a vertical sectional view through the rabble tooth shown in FIGURE 7 and taken along the line 1' 9 thereof;

FIG. 10 is a fragmentary vertical sectional view through a portion of the blade of the rabble tooth shown in FIG- URE 8 and taken along the line Iii-10 thereof;

FIG. 11 is a bottom plan view of the scraping edge of the blade of the rabble tooth shown in FIGURE 8 as viewed along the line 11-11 thereof;

FIG. 12 is a side elevation view of an alternate satisfactory form of an insert for the rabble tooth of the type shown in FIGURES 7-11 incorporating alternate satisfactory apertures therethrough for effecting an interlocking of the insert with the overlying cast iron structure, and

FIG. 13 is a fragmentary vertical sectional view illustrating the positioning of the wear-resistant insert in a sand mold prior to casting molten iron therearound.

Referring now in detail to the drawings and as may be best seen in FIGURES 1 and 2, a typical multiplehearth rotary type furnace is illustrated to which the improved rabble tooth construction comprising the present invention is applicable. The multiple-hearth furnace comprises .an outer cylindrical wall indicated at 26 having a plurality of annular-shaped hearths 22 of a refractory material mounted therein in vertically spaced increments. Conventionally, multiple-hearth furnaces of this general type are built having an internal diameter ranging from about 6 feet to about 22 feet and may include from about 4 to about 16 hearths therein depending upon the specific type of material being processed.

A tubular shaft 24 is rotatably positioned within and extends vertically upwardly through substantially the center of the multiple-hearth furnace to which rabble arms 26 are removably secured and extend radially outwardly therefrom at a position spaced above each of the hearths 22. The tubular shaft 24- and the rabble arms 26 are provided with suitable conduits or passageways such as the passageways 28 and 3b in the rabble arms 26 for circulating a cooling fluid therethrough such as air, for example. The number of rabble arms 26 positioned above each hearth 22 can be varied depending on the particular process operation being performed. In the specific multiple-hearth furnace shown in FIGURES 1 and 2, four rabble arms 26 are positioned above each hearth 22 and rotate in a plane substantially parallel thereto in response to rotation of the tubular shaft 24.

The lower portion of each of the rabble arms 26 as best seen in FIGURE .1, is provided with a longitudinally extending dovetail shaped groove 32 in which a plurality of intermediate rabble teeth 34 and in some cases, an end or terminal rabble tooth 36 are removably secured. The lower edges of the rabble teeth 34 and 36 extend downwardly to a point contiguous to the upper surface of the hearth 22 on which the material being processed is disposed. The blade portions of the rabble teeth as shown in FIGURES 1 and 2 are angularly offset with respect to the circular path of their rotation and accordingly, are effective to cause radial movement of the material inwardly or outwardly along the hearth from which it eventually cascades downwardly to the hearth therebclow and is thence moved in the opposite direction by a similar rabble arm assembly positioned thereabove. The rabble teeth can accordingly be made in both left-hand and right-hand versions or can be made symmetrical and reversible to achieve the desired direction of movement of the material.

In FIGURE 1, for example, the blade portions of the rabble teeth 34 and 36 are oriented so as to effect a radial outward movement of the material disposed on the upper hearth so that it finally falls through a peripheral chute 33 along the outer edge of the upper hearth down on the hearth therebelow. The blade portions of the rabble teeth 34- and 36 positioned above the lower hearth as shown in FIGURE 1 are oriented so as to in turn effect a radially inward movement of the material thereon until it drops to the hearth therebelow through a central chute 40 positioned adjacent to the periphery of the tubular shaft 24. Conventionally, the rabble teeth on one rabble arm are radially offset from the rabble teeth on the next adjacent arm about one-half pitch corresponding to about one-half the radial width of a rabble tooth so that the material on the hearth is moved about one-half pitch by successive arms. When four rabble arms are employed the rabble teeth on alternate arms can be offset one-half pitch or can be offset one-quarter pitch on each arm. A one-half pitch arrangement is illustrated in the drawings wherein an end rabble tooth 35 is ailixed to the ends of alternate arms. In accordance with this technique the material being processed progressively cascades downwardly back and forth across the annular-shaped hearths until the lowermost hearth is attained from which the processed material is discharged. During the downward cascading movement of the material, an upward countercurrent circulation of hue gas is maintained to effect the desired chemical reaction or roasting of the material.

It will be apparent from the foregoing that the lower edges of the blade portions of the intermediate rabble teeth 34 as well as the lower edge and vertical side edges of the blade portion of the end rabble teeth 36 are subjected to a very abrasive action by the material on the hearth which heretofore has effected an extremely high rate of wear thereof necessitating frequent replacement. In accordance with the composite rabble tooth construction comprising the present invention, the edges of the rabble teeth subjected to high abrading conditions are provided with a hard abrasive resistant insert which has resulted in a substantial increase in the useful life of the rabble teeth necessitating less frequent replacement thereof while at the same time preventing an objectionable buildup of material on the upper surfaces of the hearths and along the side walls as heretofore occasioned by excessive wear of the rabble teeth.

A composite intermediate type rabble tooth 34 constructed in accordance with the preferred embodiments of the present invention is illustrated in FIGURES 3 through 6 and a terminal or end rabble tooth construction 36 is shown in FIGURES 7 through 12. The rabble tooth 34 as shown in FIGURES 3-6 inclusive, comprises a body 42 including a blade 44 integrally affixed to and depending from a horizontal flange or web 46. The upper surface of the web 46 is provided with an upwardly projecting tongue 48 having a dovetail-shaped cross section which is adapted to be disposed in engaging relationship in the dovetail-shaped groove 32 along the lower surface of the rabble arm 26 as shown in FIGURE 1. As will be noted in FIGURE 3, the blade 44 extends diagonally of the center axis of the web 46 by virtue of which angular offset the material on a hearth is moved inwardly or outwardly, as the case may be, in response to movement of the rabble blade in a circular path thereabove. The leading edge and trailing edge of the blade 44 as best seen in FIGURES 3 and 6 are formed with a chamfer 50 extending therealong to impart a cutting action to the blade as it is advanced into the material on the surface of the hearth.

The lower downwardly projecting portion of the blade 44 as shown in FIGURES 4 and 5 is provided with a rectangularly shaped insert 52 disposed centrally within and extending for substantially the entire length of the blade to a point adjacent to the chamfered leading and trailing edges thereof. The insert 52 is preferably of a wedgeshaped or tapered cross section being narrowed at a point adjacent to the scraping edge of the tooth and becoming progressively thicker toward the inner edge thereof, as best seen in FIGURE 13, providing thereby positive retention of the insert within the tooth, in spite of any slight contraction of the insert relative to the overlying material during the casting and use of the rabble tooth. The degree of taper is not critical and tapers as small as about of an inch along a 3-inch depth have been found to be eminently satisfactory. The insert 52 is provided with one or more apertures such as the circular aperture 54 therethrough for integrally securing the overlying side portions of the rabble tooth together preventing a spreading thereof and for further interlocking the insert with the body 42 forming therewith a composite blade construction. The edge of the insert 52 is exposed along a scraping edge or lower edge 55 of the blade which is disposed in abrasive contact against the hot material on the hearth being processed. The lower projecting portion of the insert 52 projecting below the lower edge 55 of the blade 44 as indicated in phantom at 56 in FIGURES 4 and 5 is employed in accordance with the method of making the rabble tooth as will be subsequently described and is removed before placing the rabble tooth in operation.

The insert 52 may comprise any hard wear-resistant material which is characterized as having a high resistance to abrasive wear, is resistant to thermal shock, has good thermal conductivity properties, has a relatively low coefiicient of thermal expansion, is relatively hard and tough, and preferably is chemically resistant to chemical attack by molten iron. A number of materials which can be satisfactorily employed and which possess most of these properties include silicon nitride bonded, silicon carbide, ceramic bonded silicon carbide, alumina, boron carbide, magnesium silicate, zircon, and the like. A further important characteristic of the material of which the insert is comprised is that it be available or formable into a relatively closely dimensioned shape. The foregoing materials may broadly be classified as high temperature nonmetallics. A list of the more typical properties thereof are set forth in the following table:

bonded silicon carbide inserts can be satisfactorily employed in the formation of composite rabble teeth of the type comprising the present invention in accordance with the manufacturing technique subsequently to be described, in spite of its relatively poor chemical resistance to molten 1ron.

The silicon nitride bonded silicon carbide insert can be formed in a relatively dense accurately dimensioned slab of the configuration as shown in FIGURES 4 and 5 by molding and bonding in situ the silicon carbide particles with the silicon nitride bonding agent during firing at an elevated temperature effecting a tenacious bond therebetween. Typical inserts made in accordance with this technique have a porosity of about 8% and have a surface structure which is characterized by a smooth, close grained texture similar to the finish of cast iron. The molding operation of the carbide insert can be controlled so as to provide the desired taper and one or more suitable apertures such as the aperture 54 therethrough as well as a pair of parallel V-shaped score lines 58 extending longitudinally of the insert forming a division between the main body portion of the insert 52 and the lower projecting portion indicated at 56. An alternate configuration of the hard wear-resistant insert is illustrated in FIGURES 7-12 as employed in the terminal or end type rabble tooth 36. As shown in these figures, the rabble tooth 35 comprises a body 60 comprising a blade 62 which depends from and is integrally affixed to a horizon-tal flange or web 64 from the upper portion of which a tongue 66 projects having a substantially dovetailed configuration for securing the rabble tooth within the dovetailed groove 32 along the lower surface of the rabble arm 26. The web 64 is formed with an enlarged shoulder or rib 68 along the outwardly projecting edge thereof for increased reinforcement and for extending the height of a side cutting edge indicated at 70 to a point about the height of the rabble arm to which it is affixed. The blade 62 extends diagonally with respect to the axis of the dove tail tongue 66 as shown in FIGURE 7 terminating at its side cutting edge 70 which constitutes the outermost projecting portion thereof. The angularity of the blade relative to the web can be varied so as to provide a righthand or left-hand rabble tooth consistent with the direction of the rabble arm to provide the desired scraping action.

An insert 72 of a general triangular shaped configuration is embedded within the blade 62 and is exposed along the side cutting edge "/0 thereof as well as along the lower edge indicated at 74 from which a projecting portion Comparative properties of high temperature non-metallics Silicon- Ceramic 99% Nitride Bonded A1 0 Zircon IvlgO-Si0z Bonded SiG SlO Thermal Conductivity, Btu/(hm, so. ft., li)./in 113. t 109. 10.3. Coetlivieut of Thermal Expansion in./(in.," C-.), l0 4.4 4. 4. 11.0. Mohs Hardness. 9. 6 9. 6.. 7. 5. Modulus of Rupture, p.s.i. 5, G40 2, 000 600. Thermal Shock Resistance Excellent... Excellent... Fair. Abrasion Resistance Excellent..- Excellent... Good Chemical Resistance to Molten Iron- Poor. Poor. Good Of the materials as set forth in the foregoing table, silicon nitride bonded silicon carbide has been found to be particularly satisfactory and constitutes the preferred material for the purposes of the present invention. Ceramic bonded silicon carbide on the other hand, also provides for a satisfactory insert material but has the disadvantage that it is not as dense as and is not as closely dimensioned in small sections as comparable inserts made of silicon nitride bonded silicon carbide. Boron carbide also possesses exceptional resistance to abrasion but its 'relatively high cost constitutes a disadvantage in its use. 'As will be further noted in the above table, silicon nitride edge 7% and the lower edge 74 becoming progressively lower edge 74 in the specific terminal type rabble tooth 36 shown are disposed at substantially a right angle to each other. The insert 72 is preferably tapered for providing a positive mechanical interlock thereof within the surrounding body of the rabble tooth in a manner as hereinbefore described in connection with the insert 52 of the intermediate rabble tooth 34. The taper is controlled so that the insert 72 is narrowest along the side thicker toward the inner edges thereof. The insert 72 is formed with an aperture 78 extending therethrough for integrally securing the overlying side portions of the blade together holding them in firm contact against the tapered side surfaces of the insert 72.

The insert 72 as employed in the terminal rabble tooth 36 as in the case of the insert 52 employed in the intermediate rabble tooth 3- l is positioned centrally of the overlying surfaces of the blade and is interlocked therein. Alternate satisfactory configurations of the apertures through the inserts are illustrated by the keyhole shaped apertures or slots 80 in an insert 72a as shown in FIG- URE 12. The keyhole shaped slots do as shown extend from the interior of the insert and are disposed in communication with one edge thereof and are filled with metal of which the body of the rabble tooth is comprised during the casting operation forming therewith an interlocking integral network. The keyhole slots 80 are oriented so as to interrupt fracture lines and incipient cracks in the insert preventing further progression thereof. Even when fracture of the insert occurs, the fragments or resultant pieces of the tapered insert will nevertheless be retained in mechanical interlocked relationship within the body of the tooth thereby assuring continued abrasion protection of the edges of the blade in contact with the material being processed. It will be noted that the insert 72 as shown in FIGURES 8l0 as well as the insert 72a shown in FIGURE 12 is provided with a pair of inwardly extending V-shaped score lines indicated at 82 which are disposed so as to correspond substantially to the lower edge 74 of the rabble tooth 36 facilitating the breaking and removal of the projecting portion '76 prior to use of the tooth.

The metal of which the body of the rabble teeth 34, 36 is comprised includes conventional cast iron and preferably a ductile iron composition such as nodular cast iron having properties corresponding to those as set forth in the American Society of Testing Materials, ASTM Specification A-399-55. The greater toughness and duc tility of nodular cast irons conforming to the aforementioned specification provides for greater strength of the rabble tooth and improved support for the relatively fragile, hard, abrasion-resistant insert disposed within the blade portion of the tooth.

The composite rabble tooth comprising the present invention as shown in FIGURES 3-12 can readily be formed by positioning the insert centrally in a suitable sand mold and pouring a molten iron into the mold enabling passage thereof through the interlocking apertures in the insert and solidification therein and therearound providing for a firm interlocking of the insert within the blade. Since many of the hard, abrasion-resistant materials which can be satisfactorily employed for forming the insert and particularly the silicon nitride bonded silicon carbide material are susceptible to chemical attack by molten iron, the design of the blade of the rabble teeth as well as the pouring operation must be carefully controlled so as to avoid any appreciable chemical attack and degradation of the insert. Satisfactory rabble teeth can be obtained by pouring the molten iron at the lowest temperature possible consistent with that which still will enable attainment of satisfactory castings while controlling the thickness of the blade so that the sections of the molten iron in contact with the surface of the insert will chill and solidify quickly decreasing the duration of time during which the surfaces of the insert are exposed to molten iron. The chilling of the molten iron on the surfaces of the insert is facilitated by the high thermal conductivity of the material of which the insert is comprised particularly silicon nitride bonded silicon carbide inserts which are relatively susceptible to chemical attack but which possess good thermal conductivity properties.

FIGURE 13 illustrates a typical arrangement of centrally positioning a hard wear-resistant insert such as the insert 52 employed in the rabble tooth 34 shown in FIG- URES 3-6. As shown in this drawing, a sand mold generally indicated at 84 is provided with a cavity 86 corresponding to the configuration of the blade portion and the remaining upper portion (not shown) of the body of the rabble tooth. The lower portion of the cavity 86 is provided with a centrally disposed longitudinally extending slot 88 which is adapted to slidably receive and firmly retain the projecting portion 56 maintaining the insert 52 upright and positioned substantially centrally of the cavity 86. The depth of the slot 88 is controlled so that the score lines 58 separating the lower projection portion 56 and the principal portion of the insert 52 are disposed at a point corresponding to the base of the cavity which in turn corresponds with the lower scraping edge of the blade of the rabble tooth.

Since the insert comprises a relatively porous material, it is preferred to pour the molten iron substantially immediately after the insert has been positioned in the locating slot 88 of the mold to avoid any appreciable degree of moisture absorption by the insert. Greater latitude in the criticality of pouring the molten iron is provided when substantially dry sand molds such as so-called shell molds, for example, are employed which are substantially moisture free and wherein the tendency of the insert to absorb moisture is substantially eliminated.

Eminently satisfactory results have been obtained by employing a silicon nitride bonded silicon carbide insert positioned in a mold of the type shown in FIGURE 13 into which a molten ductile iron of an -6003 composition was poured at temperatures ranging from about 2400 to about 2700 F. After cooling and solidification of the molten iron, the resultant composite rabble tooth was removed from the mold and the lower projecting portion 56 as indicated in FIGURE 13 was broken away therefrom along the score lines 58 resulting in a rabble tooth 34 of the general type shown in FIGURE 4. Similarly, terminal type rabble teeth 36 of a configuration shown in FIGURES 7 through 11 have been satisfactorily cast under similar conditions employing silicon nitride bonded silicon carbide inserts having a slight inwardly diverging taper and a configuration as shown in FIGURES 8 and 12.

While it will be apparent that the preferred embodiments of the invention disclosed are well calculated to fulfill the objects above stated, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the proper scope or fair meaning of the subjoined claims.

What is claimed is:

1. A rabble tooth comprising an elongated composite blade formed with at least one scraping edge therealong, a hard wear-resistant slab-shaped insert disposed within and extending substantially the entire length of said scraping edge to a point spaced inwardly therefrom, said insert exposed along said scraping edge and forming a substantially continuous working edge, said insert including at least one aperture therethrough through which the material of said blade extends mechanically interlocking said insert within said blade, and means on said blade for removably securing said rabble tooth to a rabble arm.

2. A rabble tooth comprising a body of nodular cast iron including an elongated composite blade formed with at least one scraping edge therealong, a hard wear-resistant slab-shaped insert disposed within and extending substantially the entire length of said scraping edge to a point spaced inwardly therefrom, said insert exposed along said scraping edge and forming a substantially continuous working edge, said insert comprising a material selected from the group consisting of silicon nitride bonded silicon carbide, ceramic bonded silicon carbide, alumina, boron carbide, magnesium silicate, and zircon; said in sert formed with at least one aperture therethrough through which said nodular cast iron of said blade extends effecting mechanical interlocking between said in- S?) sert and said blade, and means on said body for removably securing said rabble tooth to a rabble arm.

3. A rabble tooth comprising a body of a nodular cast iron including an elongated composite blade formed with at least one scraping edge therealong, a hard wear-resistant slab-shaped insert of a silicon nitride bonded silicon carbide disposed within and extending substantially the entire length of said scraping edge to a point spaced inwardly therefrom, said insert exposed along said scraping edge and forming a substantially continuous working edge, said insert formed with at least one aperture therethrough through which said nodular cast iron of said blade extends effecting mechanical interlocking of said insert within said blade, and means on said body for removably securing said rabble tooth to a rabble arm.

4. A rabble tooth comprising a body of a nodular cast iron including an elongated composite blade formed with a pair of scraping edges positioned at substantially a right angle to each other, a hard wear-resistant slabshaped insert disposed within and extending substantially the entire length of said pair of scraping edges to a point spaced inwardly therefrom, said insert exposed along said scraping edges forming substantially continuous wear-resistant working edges, said insert formed with at least one aperture therethrough through which said nodular iron of said blade extends eifecting a mechanical interlocking of said insert within said blade, and means on said body for removably securing said rabble tooth to a rabble arm.

5. A rabble tooth comprising a body of a nodular cast iron including a Web and an elongated composite blade integrally afiixed to and depending from said web, said blade formed with at least one scraping edge therealong, a hard wear-resistant slab-shaped insert disposed centrally within said blade and extending for substantially the entire length of said scraping edge to a point spaced inwardly therefrom, said insert exposed along said scraping edge forming a substantially continuous wear-resistant working edge, said insert formed with at least one aperture therethrough through which said nodular cast iron of said blade extends mechanically interlocking said insert within said blade, and means on said web for removably securing said rabble tooth to a rabble arm.

6. A rabble tooth comprising a body of a nodular cast iron including a horizontal rectangular shaped web and an elongated rectangular shaped composite blade integrally afiixed to and depending from said web, said blade formed with a scraping edge along the lower edge thereof, a hard wear-resistant insert of a rectangular configuration and of a tapered cross section being narrowest at a point adjacent said scraping edge disposed centrally within and extending for substantially the entire length of said scraping edge to a point spaced inwardly therefrom, said insert comprising a material selected from the group consisting of silicon nitride bonded silicon carbide, ceramic bonded silicon carbide, alumina, boron carbide, magnesium silicate, and zircon; said insert formed with at least one aperture therethrough through which said nodular cast iron extends integrally securing the overly- Th ing side portions of said blade together at a point intermediate the ends thereof and mechanically interlocking said insert within said blade, and means on the upper surface of said web for removably securing said rabble tooth to a rabble arm.

7. A rabble tooth comprising a body of a nodular cast iron including a rectangular shaped web and an elongated rectangular shaped composite blade integrally ailixed to and depending from said web, the edge of said blade disposed opposite said web comprising a scraping edge, a rectangular shaped insert having a tapered cross section being narrowest at a point adjacent said scraping edge of a hard wear-resistant silicon nitride bonded silicon carbide disposed centrally within and extending for substantially the entire length of said scraping edge, said insert formed with at least one aperture therethrough through which said nodular cast iron of said blade extends integrally securing the overlying side portions of said blade together at a point intermediate the ends thereof and effecting a mechanical interlock of said insert within said blade, and engaging means projecting upwardly from and integrally affixed to said web for removably securing said rabble tooth to a rabble arm.

8. A rabble tooth comprising a body of a nodular cast iron including a web and a substantially rectangular shaped composite blade integrally affixed to and depending from said web, said blade formed with a pair of scraping edges disposed at substantially a right angle to each other, a hard wear-resistant insert of tapered cross section being narrowest at a point adjacent each of said scraping edges and progressively increasing in thickness inwardly therefrom, said insert disposed centrally within said blade and extending along substantially the entire length of said scraping edges thereof, said insert comprising a material selected from the group consisting of silicon nitride bonded silicon carbide, ceramic bonded silicon carbide, alumina, boron carbide, magnesium silicate, and zircon; said insert formed with at least one aperture therethrough through which said nodular cast iron of said blade extends mechanically interlocking said insert within said blade, and engaging means projecting upwardly and integrally afiixed to said web for removably securing said rabble tooth to a rabble arm.

References Cited by the Examiner UNITED STATES PATENTS 1,234,408 7/17 Stout 259134 1,340,916 5/20 Sayre a 259126 1,733,352 10/29 MacMichael 259-444 1,757,286 5/30 Banton 259144 2,743,495 5/56 Eklund 22202 2,872,715 2/59 Bean 22202 FOREIGN PATENTS 82,266 8/ 5 3 Norway. 201,216 7/ 23 Great Britain.

WALTER A. SCHEEL, Primary Examiner.

JAMES H. TAYMAN, Examiner. 

1. A RABBLE TOOTH COMPRISING AN ELONGATD COMPOSITE BLADE FORMED WITH AT LEAST ONE SCRAPING EDGE THEREALONG, A HARD WEAR-RESISTANT SLAB-SHAPED INSERT DISPOSED WITHIN AND EXTENDING SUBSTANTIALLY THE ENTIRE LENGTH OF SAID SCRAPING EDGE TO A POINT SPACED INWARDLY THEREFROM, SAID INSERT EXPOSED ALONG SAID SCRAPING EDGE AND FORMING A SUBSTANTIALLY CONTINUOUS WORKING EDGE, SAID INSERT INCLUDING AT LEAST ONE APERTURE THERETHROUGH THROUGH WHICH THE MATERAIL OF SAID BLADE EXTENDS MECHANICALLY INTERLOCKING SAID INSERT WITHIN SAID BLADE, AND MEANS ON SAID BLADE FOR R 